Synthesis and characterization of new strontium iron (II) phosphates, SrFe2 (PO4)2 and Sr9Fe1.5 (PO4)7

New strontium iron(II) orthophosphates, SrFe2(PO4)(2) and Sr9Fe1.5(PO4)(7), were synthesized by the solid state method at 1170 K. They were characterized by X-ray powder diffraction, thermal analysis in air, magnetic susceptibility, and the IR and Mossbauer spectroscopy methods. Mossbauer spectroscopy confirmed the presence of only Fe2+ cations in both compounds. SrFe2(PO4)(2), crystallizes in a monoclinic system with a = 10.5376(7) Angstrom, b = 6.8529(4) Angstrom, c = 9.3658(6) Angstrom, beta = 109.498(5)degrees. The crystal structure of Sr9Fe1.5(PO4)(7) was refined by the Rietveld method: space group R-3m; Z = 3; a = 10.6102(1) Angstrom, c = 19.7135(1) Angstrom with R-wp = 3.00% (S = 1.51), R-p = 2.35%, R-B = 3.44%, R-F = 1.72%. A second-harmonic-generation study showed that the structures of these two compounds have a center of symmetry. The crystal structure of Sr9Fe1.5(PO4)(7) is related to that of beta -Ca-3(PO4)(2) and alpha -Sr-3(PO4)(2), but is topologically more similar to that of beta -Ca-3(PO4)(2). Tetrahedra P1O(4) and strontium cations in the M3 sites are disordered in the structure of Sr9Fe1.5(PO4)(7). The octahedral M5 site is fully occupied and the M4 site is quarter-occupied by iron cations. SrFe2(PO4)(2) and Sr9Fe1.5(PO4)(7) are stable in air up to 750 and 810 K, respectively. (C) 2001 Academic Press.